From:   Chuck French/RTP/USEPA/US  

To:   "King, Heidi R." <Heidi_R._King@omb.eop.gov>

Cc:   Amy Branning/DC/USEPA/US@EPA, Paula VanLare/DC/USEPA/US@EPA, Sharon

Nizich/RTP/USEPA/US@EPA, Steve Fruh/RTP/USEPA/US@EPA  

Date:   04/09/2010 04:39 PM  

Subject: RE: Interagency comments under EO 12866 on the Gold Ore Processing

NESHAP Draft NPRM

Hi Heidi,

I recall you also wanted more information on the CEMs. Attached below is a draft

chapter on Hg CEMs from a report that NESCAUM is developing. The report is still

draft and is currently undergoing a peer review. Nevertheless, NESCAUM gave us

permission to share this draft with you. Please keep in mind that this is a

preliminary draft report that is currently under review by EPA, and that the text

and conclusions are subject to change before the final report is released.

They expect the final report to be completed by July 2010.Let me know if you have

questions.

Thanks.

Chuck

(See attached file: Draft‐chapter‐by‐NESCAUM‐on‐CEMS‐April‐2010.doc)

  
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Preliminary DRAFT chapter on CEMS – by NESCAUM – for Review 

3 Continuous Mercury Monitoring Technologies

There are two forms of continuous mercury monitors (CMMs), devices that continuously 

sample mercury emissions and provide a measurement of mercury emissions over time. They 

are 

• Analyzer-based mercury continuous emission monitoring systems (CEMS); and 

• Sorbent traps (STs). 

Hg CEMS are analyzers that continuously sample the stack gases and provide an 

indication of mercury emissions concentration in the stack within minutes of the gas being 

sampled. In this respect, the Hg CEMS provide information to the operator about mercury 

emissions on a near real-time basis, or at least in a timely fashion that provides emissions data 

for a particular hour or day. The total mass emissions rate of mercury is determined by 

multiplying the measured concentration of mercury in the flue gas times the measured flue gas 

flow rate. 

Sorbent traps provide an indication of cumulative mercury mass emissions over a period 

of time that could be hours, days, or even weeks. Gas is pulled through a set of sorbent media 

traps filled with sorbent that captures mercury. The gas flow rate drawn through the sorbent is 

varied at a rate that is in proportion to the flue gas flow rate, and must be isokinetic if there is 

significant particulate. After a period of time, the sorbent traps are removed and replaced. The 

used sorbent traps are analyzed for total mercury captured by the ST. This can be used to 

estimate the average mercury emission rate over that period of time. 

This chapter focuses on methods that are intended for permanent installation and 

continuous monitoring of mercury emissions. For continuous monitoring and reporting for the 

purpose of regulatory compliance there are requirements imposed that assure the reliability and 

accuracy of the measurement. The same technologies as discussed in this chapter have been 

used for short term test programs spanning hours, days, or even weeks. Many of the 

requirements that are discussed in this chapter that are necessary for a method designed for 

compliance purposes would not be necessary for the same method when intended for a shortterm test. 

1.1 Hg CEMS Technology Overview

There are currently two principal suppliers of Hg CEMS technology, ThermoFisher 

Scientific and Tekran Instruments. These two suppliers have supplied nearly every Hg CEMS 

ordered for utility power plants. This chapter, therefore, focuses on the technologies offered by 

these two companies. 

Each company has been working to address the specific challenges associated with 

measuring mercury in flue gas. Some of these challenges include: 

• The presence of mercury in small concentrations in power plant flue gas. 

Mercury is present in much smaller amounts than typical criteria pollutants, i.e., 

concentration measured in parts per trillion versus parts per million. Although 

ambient monitors are capable of measuring mercury concentrations to much lower 

levels than what is experienced in power plant flue gases, some of the particular 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

characteristics of mercury and power plant flue gas – such as interferences and 

reactivity - make the concentration of mercury in power plant flue gas especially 

difficult to measure at such low levels. 

• Ionic mercury is a reactive material that tends to react with or adhere to other 

materials, and thereby may be lost in sample handling without special 

precautions. 

• Elemental mercury tends to form an amalgam with metals. So, to avoid loss of 

sample, metal surfaces in the sampling system must be coated and temperature 

maintained 180 0C. 

The challenge faced by the Hg CEMS suppliers has been to design a monitoring system 

that is reliable while also addressing the above difficulties that are unique to mercury 

measurement in flue gas. 

1.1.1 Hg CEMS Suppliers

1.1.1.1 ThermoFisher Scientific Hg CEMS

ThermoFisher Scientific has shipped more than 450 Hg CEMS.28 Figure 3-1 shows how 

the ThermoFisher Hg CEMS is configured. The key components are the Sample Extraction & 

Conversion system, the Probe Controller, the Cold-Vapor Atomic Fluorescence (CVAF)

Analyzer, and the Calibration System. 

Sample Extraction and Converter 

Flue gas is sampled from the stack by the probe and is typically diluted at a 40:1 ratio 

with air (or in some cases with nitrogen). Within the probe is a converter that converts oxidized 

mercury, which is mostly in the form of HgCl2, to elemental mercury. Therefore, all of the 

mercury sent into the lines to the analyzer is in the form of elemental mercury. This makes the 

sample somewhat easier to transport and also puts the mercury in a form that the analyzer is 

capable of measuring. The sample line from the probe to the analyzer must be heated to 1200C 

(2480F) in order to ensure that sample integrity is preserved. The probe also provides the means 

to convert elemental Hg gas from the calibrator to oxidized mercury for the purpose of system 

integrity checks. 

Probe Controller 

The Probe Controller sends signals to the probe that control the oxidized Hg to elemental 

Hg converter and the elemental Hg to oxidized Hg converter, nitrogen supply and other functions 

performed within the probe. It also controls probe and sample line temperature. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Figure 3-1. ThermoFisher Instruments Mercury Freedom System.28

CVAF Analyzer 

Within the analyzer the sample is continuously measured using CVAF. In the CVAF 

method, free mercury atoms (elemental mercury) in a carrier gas are excited by an ultraviolet 

light source. The excited mercury atoms re-radiate their absorbed energy (fluoresce). The 

fluorescence intensity is related to the amount of mercury present. The technique is sensitive, 

selective, and linear over a wide range of concentrations; however, molecular gases (such as 

oxygen and nitrogen) present in the carrier gas quench the fluorescence signal, which reduces 

instrument sensitivity. 

The analyzer provides a measure of total mercury and cannot speciate mercury on its 

own. In order to measure mercury speciation, it is necessary to take two measurements – one 

with the converter in the probe operating (for total mercury) and another with the converter 

bypassed and with HgCl2 removed from the gas (to arrive at elemental Hg). The difference 

between the amount of total mercury and the amount of elemental mercury equals the amount of 

oxidized mercury. When used for regulatory purposes, CEMS are not typically operated in 

speciating mode, but rather used to measure total mercury only. 

The gas in the analyzer is maintained at a vacuum to suppress quenching and improve 

sensitivity. Sensitivity of the analyzer’s CVAF measurement can be improved roughly five-fold 

by using nitrogen as the dilution gas in the probe rather than air because oxygen has a greater 

tendency to quench the atomic fluorescence than nitrogen.28 Therefore, for low concentration 

measurements (less than 2 μg/m3

), nitrogen dilution gas is recommended by the manufacturer. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

The analyzer has digital outputs for various data protocols. The advantage of digital 

output is that, with the high degree of linearity expected from the CVAF analysis method 

combined with digital output, good linearity over a broad dynamic range is expected. 

Calibration Gases and Standards 

It is periodically necessary to verify the accuracy of the Hg CEMS using calibration gases 

that have known concentrations of both elemental and oxidized mercury. As an alternative to 

commercial, bottled calibration gases, the Hg CEMS manufacturers have been working with 

EPA and NIST on traceable methods for the instruments to produce elemental mercury 

calibration gases utilizing known relationships for mercury vapor pressure.62 The elemental 

mercury calibration gas generators for both suppliers operate by carefully controlling a sample o

Hg and dilution gas to a specific temperature and pressure that is determined by the desire

vapor pressure. In this manner, the mercury concentration of the gas exiting the calibration gas 

generator is determined by the mercury vapor pressure relationship. The calibrator output is 

NIST-traceable. 

f 

d 

It is important that the Hg CEMS measure all forms of mercury accurately. A system 

integrity check is performed to verify the accurate measurement of oxidized mercury. For 

system integrity checks it is necessary to have oxidized mercury calibration gas provided to the 

probe tip where it is drawn through the probe and through the converter and to the analyzer. To 

do this, the ThermoFisher Hg CEMS adds chlorine to the elemental Hg calibration gas at the 

probe in order to convert the elemental mercury from the elemental mercury gas generator to 

oxidized mercury. It is necessary to have a bottle of chlorine gas at the probe for this system 

integrity check. The probe has a converter that converts the oxidized mercury back to elemental 

mercury, which is then transported to the analyzer for measurement. So, for system integrity 

checks on ThermoFisher’s Hg CEMS, elemental mercury calibration gas is oxidized with 

chlorine, sent through the probe tip, and then the oxidized mercury is converted back to 

elemental mercury before leaving the probe. 

1.1.1.2 Tekran Instruments Hg CEMS

Tekran instruments has sold roughly 175 Hg CEMS to electric utilities plus additional 

systems sold for the purpose of mercury monitoring for control technology test programs, for 

research or for other purposes. The Tekran Instruments Hg CEMS, shown schematically in 

Figure 3-2, has some similarities and some significant differences with the ThermoFisher Hg 

CEMS; these will be discussed below. 

Sample Extraction 

The Tekran probe extracts sample from the stack and dilutes it with air at roughly a 30:1 

ratio. The probe is heated. The portion of the probe that is in the gas stream is made of Hasteloy 

(a corrosion-resistant metal) coated with a thin quartz layer. The probe body is made of coated 

stainless steel. The filter is titanium. Unlike the Thermo Fisher analyzer, which conditions the 

sample at the probe, the sample is sent in its sampled form to the sample conditioner, which is 

near the analyzer. The sample line from the probe to the sample conditioner is maintained at 

about 180oC (356oF) to avoid loss of ionic or elemental mercury or plating out of selenium. If 

selenium were allowed to build up in the sample line, the elemental mercury would be lost in the 

line because elemental mercury reacts with condensed selenium. An unheated line is also used 

in some cases to carry air for the probe. The unheated line does not carry sample or calibration 

gas.
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Figure 3-2. Schematic of Tekran Hg CEMS.63

 Sample Conditioner 

The sample conditioner converts the oxidized mercury to elemental mercury using a 

pyritic converter. Also, acid gases, which could induce mercury recombination reactions and 

could adversely affect analyzer reliability, are scrubbed out using deionized water. The 

conditioner therefore provides an acid-gas free stream with elemental mercury and other gases to 

the analyzer. The sample conditioner facilitates speciation by providing one of two gas streams 

to the analyzer: 1) 100% of the flue gas mercury after having converted any ionic mercury to 

elemental mercury; and 2) a sample with the ionic mercury removed so that only the elemental 

mercury from the flue gas remains. Similarly to the ThermoFisher analyzer, ionic mercury is 

determined in the Tekran analyzer by subtracting the elemental mercury concentration from the 

total mercury concentration. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Analyzer 

Both the ThermoFisher instrument and the Tekran instrument use CVAF for analysis. 

However, Tekran’s analyzer uses a gold amalgam to collect and concentrate the mercury prior to 

analysis. This also permits the mercury to be re-released into an inert gas (argon) atmosphere. 

The argon gas ensures that fluorescence is not quenched by molecular gases that are present in 

the boiler flue gas. The gold amalgam concentration method and the use of inert gas help the 

analyzer to provide high sensitivity. The analyzer consumes argon gas; one tank of argon is 

expected to last roughly 6 weeks. Like the ThermoFisher analyzer, the Tekran analyzer produces 

an output concentration of total mercury. Speciation is performed in concert with the sample 

conditioner. 

The analyzer has digital outputs for various data protocols. The high degree of linearity 

expected from the CVAF analysis method combined with digital output provides good linearity 

over a broad dynamic range. 

Calibrators 

The Tekran Hg CEMS has two calibrators – one for elemental mercury and another for 

ionic mercury. The calibrator produces a gas stream with a known concentration of elemental 

mercury in a similar manner as the ThermoFisher analyzer – by carefully controlling the 

temperature and pressure of a sample of mercury and dilution gases to provide a gas with a wellcontrolled elemental mercury vapor pressure. 

Tekran also has an oxidized mercury calibrator, the Model 3315, which generates a 

defined mixture of air and water vapor and mercuric chloride by evaporating a controlled amount 

of aqueous mercury chloride solution of known concentration into a carefully controlled gas 

volume, as shown in Figure 3-3. Since this approach starts with oxidized mercury in a NISTtraceable aqueous solution, there is assurance that only oxidized mercury is produced. Also, 

since the solution and the hardware can be NIST-traceable, a NIST-traceable amount of oxidized 

mercury can result. This method of generating oxidized mercury does require transporting the 

oxidized Hg from the generator, up a heated umbilical to the probe tip, through the probe, and 

back through a heated umbilical to the sample conditioner for the system integrity check. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Mixing Chamber

DI Water 1.0 μg/ml HgCl2

Dry Air

Misting Device

Precision

MicroPumps

Mass Flow

Meter

Hg2+

Calibration Gas

Figure 3-3. Simplified diagram of the Tekran Model 3315 Ionic Mercury Calibrator. 

1.1.1.3 Other suppliers of Hg CEMS

Three other suppliers of Hg CEMS have been identified: PS Analytical, Cemtrex and 

Ohio Lumex. These systems have primarily been used for temporary testing rather than 

permanent Hg CEMS. PS Analytical’s Sir Galahad mercury CEMS uses an amalgamation 

method combined with CVAF, the same general method as the Tekran CEMS. The Cemtrex and 

Ohio Lumex CEMS analyze the gas using the method of atomic absorption spectroscopy (AAS). 

In AAS, a light source is scanned at a wavelength where elemental mercury vapor is expected to 

absorb light. The amount of light that is absorbed is related to the amount of mercury in the light 

path. 

These analyzers may see greater use in the future. However, because of the limited 

operating experience in coal-fired boilers with these products, this chapter focuses on the results 

achieved so far with the more widely used ThermoFisher and Tekran analyzers. 

1.1.2 Approximate Costs of Hg CEMS

Both ThermoFisher and Tekran confirmed that the initial cost of a Hg CEMS is typically 

on the order of $500,000.28,64 These costs include 

• Approximately $200K for system including startup and training; 

• Another $200-$300K for site preparation. 

Costs will vary somewhat from this $500,000 figure. The heated umbilical alone is a 

significant cost item that might cost anywhere from about $10,000 for a short stack to over 

$100,000 for a very tall stack. Other factors that impact cost include the Hg CEMS located at the 

site, site conditions, and local climate. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Typical annual operating costs are more difficult to estimate, because there has been 

limited experience with the learning curve associated with the use of Hg CEMS and the actual 

quality assurance (QA) and reporting requirements are not certain. Midwest Generation 

estimates that it takes three times as many labor hours to support the Hg CEMS as traditional 

NOx and SO2 CEMS.65 Using US EPA’s CEMS cost estimating tool, there is roughly $20,000

in annual plant labor charges for a typical utility CEMS with NOx, SO2, opacity, and flow, and 

total annual costs including Relative Accuracy Test Audits (RATAs), reporting, parts and 

consumables, and other costs of about $65,000. The EPA CEMS cost model66 would sugge

that Midwest Generation’s annual labor charges have been in the range of about $60,000 for the

Hg CEMS. Due to the learning curve with these instruments, experience to date should not 

represent long-term requirements. 

st 

er 

1.1.3 Approximate number of units installed/in operation  

n shipped to date. Of 

these, m

e 

 

n’t 

n 

s 

 usage of Hg 

CMMS and these data are summarized in Table 3-1. In these states, 116 CEMS and 44 ST 

system

n of 

According to ThermoFisher, long term, the operator requirements are expected to 

approach those of the dilution CEMS that most utilities currently have installed for NOx and 

SO2.

67 This is consistent with information from Tekran that labor costs should ultimately 

approach those of the existing NOx and SO2 CEMS; but that total annual costs will be somewhat 

higher when including RATAs, reporting, parts and consumables, NIST traceability and oth

costs for a total of about $100,000 or more.68 These estimates are based upon the vacated Part 75 

requirements of the Clean Air Mercury Rule (CAMR). The actual costs of monitoring will

depend upon the requirements of any future monitoring rule. 

According to ThermoFisher, over 450 Hg CEMS systems have bee

ore than 160 systems were installed directly by Thermo Fisher, and the balance of the 

Hg CEMS were installed by CEMS system integration companies – other companies that 

purchase the analyzer from ThermoFisher and install it on the customer’s site. Because th

manufacturers, ThermoFisher and Tekran, sell spare parts, provide service, and in many cases

monitor instruments remotely, they can estimate the number of Hg CEMS in operation. Of the

total 450 Hg CEMS, ThermoFisher estimates that about 375 are actively in operation in the US.28

About 75 of the ThermoFisher systems are not active at least in part because some owners do

currently have a need to monitor mercury emissions and are choosing to avoid the cost of 

operating the systems. According to Tekran, of the 175 systems sold, approximately 140 are i

operation, 100 have completed RATA testing. The balance of the systems represent later order

that are still in the process of installation and startup.68 

In 2009, NECAUM surveyed nine states and one city as to the purchase and

s had been purchased. Most of the CEMS (73%) were installed and operating, but only a 

third of the ST systems were installed and operating. Half of the states required the collectio

Hg emissions data and information on the availability of the CMMs. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Table 3-1. NESCAUM 2009 survey of Hg CMMs in nine states and one city. 

Data Summary 

Number of States Surveyed 9 

Number of Cities Surveyed 1 

States/city collecting Hg data 5 

States collecting monitoring 

reliability/availability data 

4 

Hg CEM Prevalence 

CEMs Purchased 116 

CEMs Installed 99 

CEMs Operating 85 

ST Prevalence 

STs Purchased 44 

STs Installed 20 

STs Operating 14 

Number of units sold per vendor: 

Thermo Fisher 34+ 

Tekran 9+ 

Apex 5 

Frontier Geosciences 1 

Monitors' First Year of Use 

2006 3 

2007 1 

2008 41+ 

2009 28+ 

A CMM being active does not mean that the company is reporting data. The reason 

many companies operate Hg CMM systems (even without a current requirement to operate them 

or to report data) is because of the anticipated need to do so in the future and the benefit gained 

from the experience of operating them prior to facing the requirement. 

With few exceptions, RATA tests and other tests such as system integrity tests have been 

performed for the purpose of verifying that the systems met their contractual performance 

requirement rather than under an emissions reporting requirement imposed by a regulation. 

Therefore, the results of these tests are not publicly available unless they were published in a 

paper or other forum. Some of these published results will be shown later in this chapter. 

1.1.4 Operating Issues of Hg CEMS  

Although ambient mercury analyzers have been available and operating well for many 

years, Hg CEMS for power plant or other combustion exhaust gases are new. There are 

numerous challenges in accurately and reliably measuring mercury emissions from power plants 

that do not exist for ambient analyzers. These include: 

• Transporting the sample from the stack to the analyzer while preserving sample 

integrity; 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

• Measuring small amounts of mercury in the presence of large amounts of 

combustion gases that may interfere with the measurement; 

• Measuring mercury that may exist in multiple chemical forms; 

• Having the means to verify the accuracy of the measurement for the purpose of 

assuring that the instrument is providing reliable measure of emissions 

compliance; and 

• Doing all of this while having a system that is not unduly expensive or difficult 

for a power plant to use under the harsh industrial conditions at a power plant. 

The suppliers of mercury analyzers have made great progress in addressing these issues. 

The following paragraphs examine the key issues relating to operation of Hg CEMS, including: 

• Reliability; 

• Calibration and NIST traceability; 

• Measurement bias and bias adjustment factor; 

• Accuracy/precision at low concentrations; and 

• Availability/data substitution. 

1.1.4.1 Reliability

Because Hg CEMS are fairly new devices, there is reasonable concern regarding the 

reliability of the devices for power plant applications. In some cases product development has 

been occurring concurrently with implementation of Hg CEMS at power plants, as challenges 

associated with these applications have been encountered and design changes implemented to 

address these challenges. Recognizing that these systems have been under development and that 

some of the problems encountered a few years ago have been addressed, the focus here will be 

on issues that may continue to have an impact on reliability. 

Umbilical Failure 

The umbilical is the heated sample line(s) from the probe to the analyzer or sample 

conditioner. It is often several hundred feet long. Failure of the umbilicals has been a problem. 

Dominion Generation stated that 5 of 13 umbilicals on their Tekran systems installed in Illinois 

and Massachusetts had failed.69 These failures are mostly due to overheating. Midwest 

Generation noted similar experiences with the umbilicals to their ThermoFisher analyzers

installed on boilers in Illinois and Pennsylvania, with 6 of 13 umbilicals failing. Midwest 

Generation also noted that there were approximately 50 umbilical failures industry-wide.65 T

is roughly consistent with other sources. Tekran stated that about 10% of umbilicals have failed

which has led to new designs for umbilicals with better temperature control.64 This is consistent 

with information from Dekoron/Unitherm, the primary manufacturer of these umbilicals for bo

Hg CEMS suppliers, who reported a failure rate of 8.4% .70

The causes of umbilical failur

his 

, 

th 

es have varied. According to Dekoron/Unitherm, about 

half of 

o 

the failures were field repaired, the balance required replacement. In some cases there 

was an installation error, as installers were not accustomed to installations of several hundred t

a thousand feet of this type of heated and insulated sample line. Failure modes varied from leads

being pulled apart during installation, to short circuit of heaters, and in a small number of cases, 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

fires. Every one of the cases in which there were fires was the result of faulty installation (i.e., 

no ground fault protection despite recommendation by the manufacturer and requirements of the

National Electric Code). The other causes of the failures ranged from improper installation to 

incomplete information at the time of design, to defective heater elements (that were ultimately

replaced by the manufacturer), to lack of individual and high limit temperature control (a heater 

controller design issue). To address these causes, there have been design changes to eliminate 

the failure modes, education of installers to reduce risk of damage during installation or failure 

upon start up, and education of all parties involved to improve specification, design and 

installation to reduce risk of failure.70 In addition, many facilities have installed redunda

umbilicals to provide a backup in the event of an umbilical failure. 

As a result of the improvements made by the various companies involved in the

nt 

manufa ures 

ercise the entire sampling, conditioning, analysis, and 

calibrat nts will 

y 

 80-

ure 

h 

h 

explored more fully in th following section. 

t emissions monitors provide measurements 

, NIST standards. This is done through regular 

calibrat

erformed 

using a ST. EPA has established interim 

traceab

bined 

cture, supply and installation of the umbilicals, it is anticipated that umbilical fail

should be less troublesome in the future. 

System Integrity Tests 

System Integrity Tests ex

ion gas generating system. A shortcoming in any one of the associated compone

result in a failed System Integrity Test. According to Dominion,69 this was the most frequently 

failed QA test. Also, unlike the calibration tests, the system integrity test requires use of 

oxidized mercury calibration gases, which are more difficult to assure because oxidized mercur

is so reactive that it can be lost easily in sample streams after leaving the calibration gas 

generator (such as in the Tekran instrument), or the conversion rate of elemental to oxidized Hg 

in the probe (in the case of ThermoFisher) may be difficult to verify. Dominion recorded

150 hours of down time per Hg CEMS in 2008 as a result of failed System Integrity Tests.69 

Dominion has found that it is beneficial to perform the system integrity test manually to ens

that a technician can observe the physical process and identify problems during the test, whic

has proven to be helpful in avoiding false failures of the test. 71 This is consistent with the 

experience of Midwest Generating, who reported that their Hg CEMS do not have a good record

at passing System Integrity Checks.65 At this point, it appears that the equipment associated wit

the system integrity checks may not yet be to the point where the test can be performed in an

automated fashion with high reliability. This affects how the tests can be scheduled so that a 

technician will be available to monitor the test.71 

The difficulty of system integrity tests are closely related to development of NIST 

traceable oxidized Hg standards, which will be e

1.1.4.2 Calibration and NIST Traceability  

For compliance purposes it is important tha

with accuracy that can be traced back to verifiable

ion checks of the Hg CEMS against gases of a known mercury concentration. 

Elemental Mercury Calibration Standards 

Traceability of the elemental mercury measurement of each Hg CEMS is to be p

 calibration gas generator that is certified by NI

ility protocols for elemental gas generators, which provide procedures to: “(1) establish 

the quantitative output of elemental Hg generators; and (2) determine the expanded, com

uncertainty values of the gas standards produced by the elemental Hg generators.”62 The 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

protocol also specifies the maximum allowable uncertainty for the elemental Hg gas standards. 

Elemental mercury is also available as a bottled gas, but EPA has not yet established traceabilit

protocols for gas cylinders. 

NIST certification is achieved through an unbroken chain of comparison of one eleme

mercury calibration gas generator to another and eventually to a NIST verified standard. An 

 

y 

ntal 

interim

a 

or). 

 gas 

as 

as 

he NIST Prime gas generator needs to be 

periodi rime 

bility protocol was promulgated July 1, 2009.73 

The int rovides procedures for: “(1) establishing the 

NIST t

oxidize

l2

he 

d 

 traceability protocol has been developed to provide for this and was promulgated by US 

EPA on July 1, 2009.72 In this protocol, NIST uses isotope dilution inductively coupled plasm

mass spectrometry (ICP-MS) as the fundamental measurement to certify the NIST Prime 

elemental mercury calibration gas generator (a Tekran Model 3310 Calibration Gas Generat

This NIST Prime calibration gas generator is used to certify Vendor Prime calibration gas 

generators. The Vendor Prime calibration gas generators are used to certify User calibration

generators that are sent in to the vendors for certification. In cases where a user cannot send in 

their calibration gas generator to the vendor for certification, a Field Reference calibration g

generator, which is certified against the Vendor Prime calibration gas generator, can be used at 

the user’s site to certify the User calibration gas generator. Figure 3-4 shows how this approach 

produces an unbroken chain of measurement comparisons for certification back to a NISTverified standard. 

Certification of gas generators is valid for a period of time, and each calibration g

generator needs to be periodically recertified. T

cally recertified against ICP-MS (or another method determined by NIST), Vendor P

gas generators are periodically sent to NIST for recertification against the NIST Prime gas

generator and User and Field Reference gas generators need to be periodically recertified against 

the Vendor Prime gas generators. As a result, this will become a regular requirement for all

companies involved in mercury monitoring. 

Oxidized Hg Calibration Gas Standards 

The interim oxidized Hg NIST tracea

erim oxidized Hg gas traceability protocol p

raceability and quantitative performance of key components of HgCl2 generators; (2) 

determining the uncertainty contributed by each of these components; and (3) calculating 

certified output concentrations and expanded, combined uncertainty values for the gas standards 

produced by the generators.”62 Oxidized mercury must be used for system integrity checks. 

There are no gas cylinders for oxidized mercury, so the two CEMS manufacturers have 

developed different generators for oxidized Hg calibration gas. 

The ThermoFisher Hg CEMS starts with elemental mercury, transports it to the probe, 

oxidizes it to HgCl2 in the probe using chlorine that is added at that location, and sends the 

d Hg sample to the probe tip. Certification of the ThermoFisher oxidized mercury 

generator is related to the elemental gas generator.74 The combined uncertainty of the HgC

calibration standards is calculated based on the uncertainty of the elemental Hg generator, t

water vapor dilution (if applicable), and the chlorine dilution. In the interim oxidized Hg 

protocol, the oxidation efficiency of Thermo systems is not included in the overall combine

uncertainty calculation. However the apparent percent conversion of elemental Hg to mercuric 

chloride is calculated as part of the periodic QA (data quality) assessments.75 

For the Tekran Model 3315 oxidized mercury calibrator that uses an evaporative gas 

generator, NIST traceability may be achieved in a different manner because NIST-traceable
Draft Under Development – Review Draft – by NESCAUM – April 2010 

solution ed 

g 

actual 

s are available and it is possible to verify the precision of the metering pumps, etc., us

in the device. For a reason that is not clear, comparisons of this type of oxidized Hg gas 

generator (Tekran 3315 or any other evaporative type) against elemental gas generators have 

shown a fairly consistent difference of about 8% to 9%, with the ionic calibrator always 

producing a lower level. Because of the consistency of this difference, this type of oxidized H

calibration gas generator can be certified by NIST.76 This approach does not involve an 

measurement to verify the output concentration, but relies on the calibrated, theoretical value. 

NIST ID -ICP Mass Spec

NIST Prime gas 

generator

Vendor 1 

Prime gas 

generator

Vendor 2 Prime 

gas generator

User F gas 

generator

User C gas 

generator

User A gas 

generator User D gas 

generator

Vendor 1 

Field 

Reference gas 

generator

Vendor 2 

Field 

Reference gas 

generator

NIST

User B gas 

generator

User E gas 

generator

Figure 3-4. General approach for elemental mercury calibration gas generator certification. 

1.1.4.3 Measurement Bias and Bias Adjustment Factor

M), 

ents (up or down) may be 

made. 

ances in the market, bias adjustments are made up, but not 

down. up so 

t 

nces or to 

When a measurement device indicates a different value than a reference method (R

and that difference is consistent, the difference is regarded as bias. To correct for this difference 

and make the reported values consistent with the RM, bias adjustm

40 CFR Part 75 monitoring requirements require such adjustments, but Part 60 

monitoring requirements do not. 

When compliance is achieved under a trading mechanism, where the measurement 

provides the “currency” used in the allowance market and where an excess in emissions can be 

made up through purchase of allow

That is to say, if the RM shows a higher value than the monitor, the bias is adjusted 

that the reported emissions are higher than would otherwise be indicated by the monitor. But, on

the other hand, if the RM shows lower emissions than the monitor, a downward bias adjustmen

is not made. This is how the bias adjustment was made under the Part 75 monitoring 

requirements of the CAMR, which have been vacated. However, under a compliance 

mechanism where trading is not an option there may be an argument to make bias adjustments 

both ways rather than one way. This is because there is no ability to sell excess allowa
Draft Under Development – Review Draft – by NESCAUM – April 2010 

purchase allowances that are needed. The potential problem if there is no requirement

for low bias is that it could potentially open an opportunity for sources to deliberately adjust the

Hg CEMS to read below the reference method, while staying marginally within the percent 

relative accuracy requirements when performing a RATA. This could result in under-reporting 

of emissions.77

The issue of bias adjustment was raised in testimony before the Illinois Pollution Control

Board by representatives of electric utilities.69 The Illinois EPA subsequently agreed to remove

bias adjustme 78

 to adjust 

ir 

nts from their rule. However, for the most part the Illinois utilities are complying 

with a p

is is a 

 in 

ents is 

w 

ry concentrations because some air pollution control systems (or combinations of air 

removal of mercury that can 

thermore, some states have required 

90% re

oth 

ire system – probe, sample line, sample conditioning, etc., in the presence 

of flue 

, and an accuracy that has a relatively steady offset that is generally 

under 0 ws 

ercent reduction requirement or alternatively with a technology-based standard and 

therefore have been given the option of quarterly stack tests rather than Hg CEMS. So, th

different situation than would exist with an annual mass emission limit where the argument

favor of bias adjustments is stronger. As a result, the argument in favor of bias adjustm

not as great under Illinois’ rule. 

1.1.4.4 Accuracy/Precision at Low Concentrations

Accurate and precise mercury measurements are becoming increasingly important at lo

mercu

pollution control systems) are capable of producing high percent 

result in emissions concentrations well below 1.0 μg/m3. Fur

moval of mercury from flue gases. Accuracy relates to how close the measurement is to 

the actual number. Precision relates to how repeatable a measurement is at a given level. B

qualities are desirable. 

Because the CVAF method is capable of measuring mercury concentrations to levels far 

lower than what is experienced in the flue gas, as demonstrated in its use in ambient monitoring, 

the concern regarding accuracy and precision at low concentrations is driven by concern about 

the reliability of the ent

gas. Also, there is concern about the ability to verify these against a RM, such as Ontario

Hydro or Method 30B (STs), which also is likely to have poorer relative accuracy and precision 

at lower concentrations. 

As demonstrated in Figure 3-5, Hg CEMS have demonstrated the ability to measure 

mercury concentrations well below 1.0 μg/m3

 and to agree with the Ontario Hydro Method (the 

RM) measurements within about 0.09 μg/m3

. The trend shown indicates good precision when 

compared against the RM

.10 μg/m3

. Figure 3-6, which shows data taken from a stack with a wet scrubber, sho

that good agreement between Hg CEMS and STs has also been achieved at levels well under 1.0

μg/m3. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Figure 3-5. RATA results at low mercury concentrations.64

Figure 3-6. Low level Hg CEMS accuracy check against sorbent traps on a stack that follows a wet scrubber, 

or wet stack.64

Because the absolute difference between the RM and the Hg CEMS at low concentrations may 

be a significant percentage of the measurement due to low absolute values of the measurement, 

this can be a significant issue when making bias adjustments. In the absence of bias adjustments, 

the concern may be less critical. However, with regard to certifying accuracy under a RATA, if 

measurements are expected to be well under 1.0 μg/m3

, as shown in Figure 3-5 and Figure 3-6, 

then accuracy requirements (expressed as within X% or Y μg/m3

) will need to be considered, so 

that the allowed error band at these low levels is sufficiently large that well-running instruments 

will pass their QA requirements, yet not so great as to make the reported measurement 

meaningless. In the original requirements under CAMR, the relative accuracy requirements did 

take this into consideration and had different provisions under 1 μg/m3

. 

1.1.4.5 Availability and Data Substitution

For Hg CEMS used for compliance under a trading system, there are strict requirements 

for both availability and data substitution. Availability is a measurement of the number of hours 

in an operating period the device is providing reliable data. Data substitution is sometimes 

required for periods in which a monitored facility is operating, but the emissions monitoring 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

device is not providing reliable data (not available). Data substitution introduces assumed 

emissions data for the period of time when the monitoring device is not available. The 

monitoring requirements under the Clean Air Mercury Rule required 75% data availability. 

Even absent trading, due to the variability in mercury emissions, there is an argument for having 

high data availability. Mercury emissions are sensitive to many factors that can vary routinely 

for a power plant, such as furnace oxygen (through its impact on fly ash carbon), soot blowing, 

coal mercury levels and other factors. Figure 3-7 shows results of a RATA at Nebraska Public 

Power’s Sheldon Unit 2. Measured mercury concentrations varied by more than a factor of two 

over the period of the RATA using Hg CEMS (shown in figure as HgT) versus sorbent traps as 

the reference method (shown as 30B). As a result, data substitution can be inaccurate and 

periods of lost data should be minimized. 

There are several factors that affect Hg CEMS availability, including the following. 

• Stringency of Regulation – If QA testing requirements are stringent, then there is 

likely to be more down time since failure of a QA test will require 

troubleshooting. 

• Technician Skill Level – This will vary from plant to plant. The higher the skill 

level of the technicians, the faster they will be able to restore an instrument to 

service in the event of a failure. 

• Priority of Preventive Maintenance (PM) Program – Is the PM program at the 

plant a management priority that has adequate resources? If not, then 

maintenance in general for all equipment will be poor and uptime will suffer. 

• Spare Parts Stocking – If spares are on hand, the instrument can be restored to 

service faster than if parts must be ordered. 

According to ThermoFisher, customers that are conscientiously operating and 

maintaining the equipment, even with no current reporting requirement, have been able to keep 

the instruments operational about 90-98% of the time.28
HgT Draft Under Development – Review Draft – by NESCAUM – April 2010 = Total Mercury from Hg CEMS CEM Ave = average Hg CEMS reading during sorbent trap sample period 

30B = Sorbent Trap measurment run diff = the difference between CEM Ave and Sorbent Trap 

allowed = allowed difference for RATA 9 run mean difference = average difference for nine RATA runs 

Figure 3-7. Nebraska Public Power Sheldon 2 RATA results.79

 

 Availability is a concern for new technology. Although few states require reporting of 

availability, Massachusetts did request that the coal plants in Massachusetts report availability of 

their Hg CEMS. As shown in 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Table 3-2, the Hg CEMS at the Brayton Point station reported availability ranging from 88% to 

94%.80 Mount Tom generating station reported 98% availability.81 

Strictly speaking, the reported availability statistics in 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Table 3-2 for Hg CEMS in Massachusetts are not subject to the Part 75 requirements of CAMR 

because those requirements were vacated when the CAMR was vacated. Also, there are some 

aspects of the original Part 75 CAMR rule that have since evolved out of technical necessity, 

particularly with regard to the move to on-site calibration gas generators that will be NISTtraceable rather than use of NIST-traceable compressed gas cylinders. The reported data are not 

QA-tested by the Massachusetts Department of Environmental Protection (DEP) in exactly the 

same manner as US EPA would have done under a Part 75 requirement because US EPA – who 

has the capability to perform Part 75 data QA - lost the authority to perform data QA when 

CAMR was vacated. According to the Massachusetts Department of Environmental 

Protection,82 the Massachusetts requirements under 310 CMR 7.00 et al. are otherwise identical 

to those of the now-vacated Part 75 CAMR requirements. The units in Massachusetts must 

undergo the same daily calibration checks, weekly system integrity checks, quarterly linearity 

checks, RATAs and other requirements of the now vacated Part 75, and these availability 

statistics and the good quality emissions data being generated are a strong indication of the 

industry’s efforts and ability to operate Hg CEMS. 

 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Table 3-2. Availability of Hg CEMS at Dominion Brayton Point power plant in 2008.79

Quarter 1 Quarter 2 Quarter 3 Quarter 4 Overall 

Operating Hours 1,536 2,076 2,102 2,159 7,873 

Out of Service Hours 96 223 87 52 458 

Unit 1 Availability, % 93.7 89.3 95.9 97.6 94.2 

Year-to-date Availability, % 93.7 91.17 92.89 94.2 

Operating Hours 2,184 2,116 1,994 2,208 8,501 

Out of Service Hours 140 123 415 346 1,024 

Unit 2 Availability, % 93.6 94.2 79.2 84.3 88.0 

Year-to-date Availability, % 93.6 93.88 89.23 88.0 

Operating Hours 2,170 1,422 1,929 2,072 7,593 

Out of Service Hours 359 253 55 104 771 

Unit 3 Availability, % 83.5 82.2 97.1 95.0 89.8 

Year-to-date Availability, % 83.5 82.96 87.92 89.8 

Missing data are normally substituted under a Part 75 system because of the ability to 

trade mass emissions allowances. Under a rule where a percent reduction or implementation of a 

specific control technology is required, it could be argued that data substitution may not be 

necessary. On the other hand, in the case where there is a mass emissions limit, it could be 

argued that data substitution should be performed to ensure that mass emissions are not underreported. Also, data substitution for missing data provides motivation for maintaining a high 

availability of the instrument. 

1.2 Sorbent Trap Method (EPA Method 30B)

1.2.1 Sorbent Trap Method Description

The sorbent trap (ST) method is an approach to measuring total mercury emissions over a 

period of time. The continuous measurement method was addressed by Appendix K of the Part 

75 requirements of the now vacated CAMR. EPA Method 30B has replaced Appendix K and is 

a reference method that can be used for RATAs as well as for monitoring. The ST method is 

pictured in Figure 3-8. Gas is drawn through two separate traps concurrently. Two traps are 

necessary for the purpose of quality assurance, and agreement must be within prescribed levels 

or the sample fails QA. The two traps can be installed within the same probe or in separate 

probes. The mercury traps must also be installed in the probe such that gas is pulled directly into 

the traps. The method assumes that there is not a significant concentration of particulate matter 

in the flue gas. A common trap sorbent material is activated carbon impregnated with iodine or 

another halogen like bromine. For continuous monitoring each trap must have the sorbent 

configured into three sections that can be tested independently. The first section is for primary 

mercury collection. The second section is to provide an indication of mercury breakthrough. If 

there is too much breakthrough, the sample fails QA. The third section is for spike recovery 

testing using a spiked mercury sample gas. Analysis of the third section must show good 

recovery of the spiked sample for acceptable results. After a period of sampling, the traps are 

removed and analyzed to measure the total amount of mercury in the traps. The mercury found 

in the traps divided by the total gas flow over the period of the sampling indicates mercury 

concentration in the gas. 

The balance of the sampling system includes a moisture removal system, and dry gas 

flow pump and dry gas flow meter (or other equivalent mass flow equipment) and related 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

instrumentation. The flow meter and associated instrumentation (barometer and temperature 

indication) must be regularly calibrated to ensure that they are reporting the correct volume of 

gas. The flow meter must also have a variable flow control to maintain sample flow rate in 

proportion with load. 

Figure 3-8. Method 30B sorbent trap equipment.84 

Because of the QA requirements that could cause a sample to be rejected, most users 

replace traps every 4-7 days. This assures that if a sample is rejected for QA reasons, that only 

4-7 days of data are lost.83

The two most widely used approaches to analyzing the traps for mercury are: 1) a wet 

digestion method using atomic absorption or fluorescence analysis; and 2) thermal release with 

atomic absorption analysis. The latter approach can be done on site with the proper equipment. 

However, it has the disadvantage that the sample is destroyed, so that if there is a problem with 

the analysis it cannot be checked again. 

Sorbent traps can be used as a reference method against a CMM or as a CMM. Sorbent 

trap requirements as CMMs were defined under the CAMR as Appendix K. As a reference 

method, STs are EPA Method 30B. Since it is used as a reference method, Method 30B has 

different QA requirements than Appendix K in terms of spike recovery, etc., and is somewhat 

different in procedure. 

1.2.2 Sorbent Trap Accuracy/Precision

The ST method for measuring mercury generally has good measurement accuracy down 

to relatively low mercury concentrations well under 1 μg/m3

. This can be influenced by sample 

time, trap size and other parameters that the user can control depending upon their needs. Figure 

3-9 (presented in Ref. 85) and Figure 3-10 (presented in Ref. 86) show results from six different 

RATA’s from six different power plants, in each case a, b and c showing results for low 

concentrations (under 1 μg/m3

), moderate concentrations (about 1 μg/m3), and higher 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

concentrations (over 1 μg/m3), respectively. Relative accuracy can be more difficult at the 

lower concentrations because there appears to be more variation in concentration on a percent of

value basis at these low levels of mercury. The sorbent trap CMM in the figure being compared

to a sorbent trap reference method (Method 30B) is either MET-80 (sold by CleanAir 

Engineering) or Appendix K. In these cases both sorbent trap methods (either Appendix K o

Method 30B) were being drawn at the same time. So, in each case there was a comparison of 

total mercury collected in the traps over the same period. 

r 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

(a)

(b)

(c)

Figure 3-9. RATA results reported by Wright and Schmid85 with (a) concentrations under 1 μg/m3; (b) 

concentrations near 1 μg/m3

; (c) concentrations over 1 μg/m3. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

(a)

(b)

(c)

Figure 3-10. RATA results reported by Wright86 with (a) concentrations under 1 μg/m3; (b) concentrations 

near 1 μg/m3

; (c) concentrations over 1 μg/m3. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

1.2.3 Approximate Costs of Sorbent Trap Systems  

The initial installed cost of a sorbent trap system is on the order of $150,000. Operating 

costs entail roughly $500 to $700 per week for traps and analysis ($26,000 to $36,000 per 

year).61,68 The cost of labor for operating the traps corresponds to about two people for half a 

day per stack every 4 to 7 days. 86 At $50/hr for labor, that is roughly another $21,000 to $36,00

per year. 

0 

1.2.4 Advantages/Disadvantages of STs When Compared to Hg CEMS  

The advantages of STs versus Hg CEMS are: 

• Significantly lower capital cost - The principal advantage of STs over Hg CEMS 

is much lower capital cost – about $150,000 versus about $500,000. 

• Simpler system to operate and maintain – Sorbent trap systems are less complex 

than Hg CEMS and don’t require as sophisticated a technical staff to support. 

However, the personnel involved in STs do need training on how to properly 

handle the traps to avoid contamination and other problems that could introduce 

errors. 

• Lower concentrations - In principal, STs may be able to measure to lower 

concentrations than Hg CEMS, but this is very methodology dependent. 

Furthermore, Hg CEMS are improving their ability to measure to low levels such 

that any advantage in this respect may be small. 

Disadvantages of STs versus Hg CEMS are: 

• No real-time, or near real-time data – This is the principal technical advantage of 

Hg CEMS over STs. The information is available in minutes, which enables the 

performance of Hg emission control processes to be monitored in near real time 

and can thereby offer advantages in Hg control. Figure 3-11 shows how Hg 

CEMS data and ST data compare at one facility equipped with a wet scrubber 

over a period of 5 days and 30 days, respectively. As shown, there was a wide 

variation in Hg emissions that is not captured by the STs. 

• Higher Labor Costs - This may not have been realized yet since Hg CEMS have 

been coming down a fairly steep learning curve, which means that owners have 

had to dedicate more hours to service and support than they ultimately should 

have to. Sorbent traps, on the other hand, will always require regular trap 

replacement and testing (typically every 4-7 days) and the labor associated with 

replacing and testing the traps is not likely to drop very much. 

• Loss of Data – If there is a problem with a ST sample, it may not be discovered 

until after the sample is completed, and when ST samples fail the quality control 

tests, the data are lost for the full sampling period. For this reason, ST systems 

have quality control procedures in place to minimize this risk, and in practice 

samples are limited to about 4-7 days. For Hg CEMS the existence of a problem 

that would cause lost data is generally apparent right away, so that corrective 

action can be immediately taken to restore the instrument to a fully functioning 

state. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

• Personnel Safety – STs require that personnel regularly go up to the sampling 

point to remove and replace sorbent traps. This creates an added concern for 

personnel safety. 

• Speciation – Although only total Hg emissions would have been reported under 

the vacated Part 75 CAMR requirements, oxidized Hg and elemental Hg do 

behave differently in terms of capture and in terms of how they behave once 

released to the environment. So, there is a benefit with respect to emissions 

control and with respect to understanding environmental impact in having 

speciated Hg emissions information. Although approaches do exist for speciated 

Hg measurements with STs, due to the reactivity of oxidized mercury, this is 

difficult. And, there is limited experience with taking speciated measurements 

with STs. 

• Process Steps - There can be many people and steps necessary to assure a good 

measurement; a mistake at any point in the process could introduce error. 

• Hazardous Wastes - Hazardous wastes can be generated from the analysis 

methods that need to be disposed of in accordance with procedures for handling 

and disposing of hazardous materials. 

• Trap Analysis - The analysis of the trap (thermal desorption or wet chemical 

analysis) destroys the sample, so that it can’t be checked in the event of an 

analysis error. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

(a)

(b)

Figure 3-11. Hg CEMS performance compared with Sorbent Traps.87

1.2.5 Availability of STs used as CMMs

Because Hg monitoring data have not been required to be reported by many states, there 

are limited data on reported availability of the method. However, information on short-term 

experience (under 6 months) using Part 75 availability requirements has shown availability 

generally over 90%. Figure 3-12 shows availability data over several months for eight different 

STs. Also, as shown in Table 3-3, year-long data from Dominion reported to the Commonwealth 

of Massachusetts on their Salem Harbor Units 1 through 3 CMMs show availability of 89% to 

96%.87
Draft Under Development – Review Draft – by NESCAUM – April 2010 

Figure 3-12. Sorbent Trap availability reported by Wright and Schmid.89

Table 3-3. Availability information for Dominion Salem Harbor Units 1, 2, 3 CMMs.87

Unit 1 Unit 2 Unit 3 

Online Operating Hours 5,922 6,209 3,620 

Monitored Hours 5,263 5,587 3,480 

Total Operating Hours of Downtime 659 622 140 

Availability 89% 90% 96% 

Missing data caused by: 

 Monitor not operated* 269 82 0 

 Monitor Malfunction/Maintenance 12 74 1 ** 29

 Trap QA/QC*** 360 429 0 

 Trap Change Out 5 17 3 

 Monitor Calibration 13 12 8 

Notes: 

* Early on in the monitoring program the monitor operating parameters (e.g., vacuum, trap flow) were not well 

ved. 

** 

r. In 

. 

 The importance of availability is illustrated by Figure 3-13, which shows variation of 

mercur

s 

enough understood to use as predictive information to change out traps in a timely manner to avoid the 

monitor shutting down on its own. Automated alerts were not sufficient. The issue has since been resol

One recurring maintenance issue caused the bulk of this downtime. The issue has since been resolved. 

*** Initially the sorbent traps were not able to sample for longer than three days due to a loss of spike recove

June 2008 the design of the traps was improved, which allowed the duration of the run to increase. 

Subsequently, data loss due to trap quality assurance/quality control (QA/QC) has been greatly reduced

y concentration over a several month period for three different boilers: bituminous and 

unscrubbed in the case of Figure 3-13a, bituminous and scrubbed in the case of Figure 3-13b, 

and PRB and unscrubbed in the case of Figure 3-13c. In each of these three cases – which are 

very different situations from the perspective of coal types or inherent mercury capture - there i
Draft Under Development – Review Draft – by NESCAUM – April 2010 

significant variation of mercury concentration over the period. Because of this variability in 

emission rate for each boiler, the lack of high CMM availability would result in much less 

confidence that the measured mercury emissions were a reliable indicator of actual mercury 

emissions over the period. 

Strictly speaking, the reported availability statistics in Table 3-3 for the STs in 

Massachusetts are not subject to the Part 75 requirements of the CAMR because those 

requirements were vacated when the CAMR was vacated. And as discussed previously, some 

changes from the original Part 75 CAMR protocol have been made out of technical necessity. In 

other words, these changes would have occurred even if CAMR were still in place. According to 

the Massachusetts DEP,82 the Massachusetts requirements under 310 CMR 7.00 et al. are 

otherwise identical to those of the now vacated Part 75 CAMR requirements, and these 

availability statistics and the good quality emissions data being generated are a strong indication 

of the industry’s efforts and ability to use STs for the continuous monitoring of mercury 

emissions. 
Draft Under Development – Review Draft – by NESCAUM – April 2010 

(a) (b)

(c)

Figure 3-13. Sorbent Trap data from Wright89 for (a) bituminous, unscrubbed; (b) bituminous, scrubbed; (c) PRB,